Maybe it was a very good act. Rash"I doesn't exactly go into detail. And even n'vi'im are not immune to trickery. Without it, arguably, Ya'akov Avinu would not have gotten his b'racha.
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WAFNov 20 '11 at 13:16

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Where do we learn that Yitzchak was a navi, and either way why would we believe that even a great man can never be deceived? Nowadays many parents are particularly open to deception when it involves their own kids, who they want to believe are good kids.
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Monica Cellio♦Nov 20 '11 at 18:03

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@MonicaCellio, Yitzchak was a navi according to Rashi to M'gila 14:1.
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msh210♦Nov 20 '11 at 18:57

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An example of a navi being taken in by trickery is in I Kings 13:18-19, where the prophet sent to warn Yeravam (the Midrash identifies him as Iddo) is misled by a false prophet who claims to be bearing a message from Hashem.
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AlexNov 20 '11 at 21:52

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@simchashatorah being a navi doesn't make one omniscient; there's only One who is. It just means that God has chosen that person to receive and retransmit particular special messages from Himself.
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Isaac MosesNov 21 '11 at 5:06

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R. Shneur Zalman of Liadi explains (in Torah Ohr, Toldos 20c) that Eisav had great spiritual potential embedded in him (ציד בפיו, lit. "something captured in his mouth") - as indeed eventually was realized in descendants of his such as R. Meir, Onkelos, and Ovadiah.

Yitzchak, then, had no illusions of Eisav's righteousness; he was well aware that it's going to take a lot of effort to extract these sparks of holiness. However, he figured that this could be accomplished by blessing Eisav, thereby infusing into him a powerful Divine "light" that would sweep up all of these "sparks" with it. The problem was that Eisav was totally unsuited for this, so if he had tried to carry out this plan, one of two things would happen: either this Divine energy would have been assimilated into Eisav's unholiness and been wasted, or it would have completely overwhelmed him. It turned out, then, that the only way to accomplish the task was to give the blessings (and the accompanying Divine energy) to Yaakov, and through him it would reach Eisav in a trickle-down effect.

Prophecy and all, man is still flesh-and-blood. To know the mind of G-d would to be G-d.

What of all the "deep spiritual sixth sense" and the like? Well I guess a really evil faker can fool that too. That's how there's balance in the world. (R' Menachem Mendel of Kotzk observes that Ethics of the Fathers tells you what differentiates students of Father Abraham from students of the villainous Bilaam. "Why talk about the students and not them?", asked the Kotzer. "Because the average person wouldn't be able to tell the difference at first glance between Abraham and Bilaam")

There are plenty of stories regarding the Chafetz Chaim (Rabbi Israel Meir Kagan, lived in Russia about a hundred years ago) which indicate that he was "tapped in" to a greater source of knowledge -- just the senses he had, or things he'd referred to. Fine. But as we're humans, it doesn't work 100% of the time. It can't. When a brilliant charlatan "discovered" some long-lost volumes of the Talmud (pdf), many rabbis were skeptical -- but the Chafetz Chaim was fooled.

The Gemara in the last chapter of Pesachim talks about certain things the way the world works; despite fancy technology, these are all still fundamentals of human civilization. One of them is "that a person can't truly know what's in another person's heart."

Now clearly Esav was a more rough-cut person than his brother, there was no doubt about that. His father figured that one brother would engage the world, and assist the other brother who would be engrossed in spirituality. It was a reasonable idea, but he didn't realize how evil Esav was -- or that Yaakov had the ability to outgrow his shell and engage the world too.

This shiur by Shmuel Braun delves into a pshat by the Ishbitzer Rebbe on this point. He says Yitzchok was aware that Eisav was pretending to be something he was not, and it was precisely because of this that he loved him.

This shiur discusses two aspects of this, listen or watch for the full explanation:

Yitzchok loved that Eisav was willing to pretend to be a higher level than he was (even if he may have fallen because of it).

The Baal HaTanya once said that those who pretend to be Chassidim will not die without actually become so.

Chalila (How to say it in English?) to think that Yizhak could have been fooled. We have no idea of the level of our Fathers, which we can only start to imagine when we learn about the leaders of previous generations. But the question has to be answered.

I just posted this answer to a related question. There is an opinion which I heard in a shiur given by Rabbi Baruch Rosenblum, that Yizhak knew that Am Yisrael will be Malechet Cohanim (a nation of Cohanim) totally involved in Torah and avodat Hashem, but for that you need a Zebulun that will take care after all the material needs of the Cohanim. He saw that Ya'akov was ideal for the job of the Cohen so he thought that Eisav would be this Zebulun and that is why he wanted to give him the blessing that dealt with what is needed to achieve this. He saw exactly how Eisav behaved and knew that this is the kind of behavior that will be needed for his mission and the blessing were intended to help him with the job.

Why is it "chalila" to think that Yitzchak was fooled by Eisav? Is it not explicit that he was indeed fooled by Yaakov?
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jakeNov 20 '11 at 22:41

@jake, I'll quote a weekly sheet of divre Tora published by my local kolel: "Was he unaware of the fact that Yaakov spent his days and nights immersed in... Torah study while Eisav roamed the fields and forests? Their respective reputations were so well known that even the young Leah... dozens of miles away heard... activities and character of Eisav (Medrash, Vayeitzei). Once the twins reached adulthood, the piety of Yaakov and the beastliness of Eisav became apparent to all. (Rashi 25:26) ... The evidence that Yitzchak was aware of Eisav's true nature is more than compelling. [continued]
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msh210♦Nov 27 '11 at 21:21

"[continued] Before sending out Eisav to trap..., he warned him not to steal it and to make sure it would be [shechted right]. This is hardly... one needs to reiterate when... pious man.... Furthermore, when Yaakov donned the hairy skins... he spoke in a refined manner and invoked the name of G-d. This made Yitzchak suspicious, for it was out of character for Eisav...."
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msh210♦Nov 27 '11 at 21:23

You're right, @jake, that it's explicit (in Rashi, not the chumash) that he was fooled by Esav. But it definitely omer darsheni.
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msh210♦Nov 27 '11 at 21:25